This invention relates generally to the delivery of helium fluid at high purity to a usage location.
When large quantities of industrial gases, such as nitrogen, oxygen, argon and hydrogen are required at a usage location, they are commonly transported from the production plant as liquified gases and transferred to a storage tank at the usage site. The pressure in the storage tank may be allowed to increase to the maximum working pressure of the storage tank by natural evaporation and by induced evaporation of a quantity of liquid in an external or internal heater. The liquified gas is withdrawn at the tank pressure for use as needed, pumped to a higher pressure if required, vaporized and warmed if desired.
Helium is not amenable to such practice. Since helium has a very low heat of vaporization and boils at a very low temperature, liquid helium pumps are not viable because of heat leak and vaporization during the pumping operation. Even during transfer by pressure differential from a transport vessel to a storage tank at a usage site, excessive vaporization of helium liquid occurs during cooling of the storage tank and associated piping with attendant excessive loss of gaseous helium.
Consequently common practice is to transport helium as a liquid to a transfill site in a liquid trailer, vaporize the liquid and compress the resulting gas into high pressure cylinders for delivery to the user site. This process often results in contamination of the helium stored in or drawn from the cylinders with trace amounts of air and moisture. If high purity helium is desired at the user site, an expensive purification system is installed to remove the contaminants.
An alternative is to transport helium directly to the usage site as liquid in an insulated vessel, allow the vessel to remain at the usage site and withdraw helium from the vessel by pressure differential as needed for use. The pressure in the vessel may be allowed to increase to the maximum working pressure of the vessel by natural evaporation and by induced evaporation with a heater. The heater may be external and require a circulation of a fluid stream of helium through the heater, or the heater may be internal such as an electrical element. In either case, additional complexity and cost are required and are disadvantageous.
As an alternative to the heater, U.S. Pat. No. 4,766,731 describes pressurization of a liquid helium container with gaseous helium from an external source by bubbling the additional gaseous helium through the cold helium fluid in the container in order to cool this additional helium to the temperature of the fluid in the container. This procedure destroys or reduces the natural temperature stratification that develops in a container of liquid helium. Also the additional gaseous helium must remain in heat exchange relation with the cold helium fluid in the container for a period of time sufficient to be cooled before fluid delivery to the user occurs. Alternatively, according to this patent, the additional gaseous helium may be passed in indirect heat exchange relationship with the cold helium fluid in the container to cool it to the temperature of the fluid in the container, and then released into the container or into the supply line leading to the usage location. This latter procedure requires a heat exchanger for the indirect contact between the container fluid and the added helium gas.
The current invention has the advantage of allowing a standard liquid helium transport vessel without modification to be used for transporting liquid helium to a usage site. Another advantage is that helium fluid may be withdrawn simultaneously with the addition of pressurizing gas. Still another advantage is that the natural temperature stratification in the vessel is maintained. Thus a lesser quantity of additional relatively impure helium gas from an external source is required to pressurize the vessel for withdrawal and supply of high purity helium fluid to a usage location. In addition, a larger fraction of the helium fluid charged into the vessel may be withdrawn from the vessel as high purity helium fluid for delivery to the user.